Heretofore, a gas detector, a human sensor, and the like have been known as optical sensor apparatuses including a light receiving element and an optical element including a reflecting layer. For example, a gas sensor described in JP 2012-220351 A (hereinafter referred to as “Document 1”) has a configuration in which a space for introducing a detection target gas is formed between a light emitting element (light emitting unit) and a light receiving element (light receiving unit), and a light guide body is arranged between the light emitting element and the light receiving element. The inside of the light guide body includes a reflecting layer (reflecting mirror) formed by a metal such as aluminum. The reflecting mirror described in Document 1 is formed by a metal plate or a metal film. The gas sensor can detect gas concentration by utilizing the fact that the gas introduced in the light guide body absorbs light having a specific wavelength.
A corrosive gas may be introduced inside the gas sensor of this type depending on the use environment. For example, if the gas sensor is used in a kitchen, sulfide and chloride may possibly be introduced inside the light guide body. Accordingly, there is concern that the reflecting layer formed by a metal will be corroded, resulting in sensitivity reduction in gas detection due to reduction of reflectance.
It is considered that a corrosion resistant layer (corrosion protective covering) is formed on a surface of the reflecting layer in order to suppress corrosion of the reflecting layer (refer to JP 2005-143943 (hereinafter referred to as “Document 2”), for example). A corrosion protective covering made of a synthetic resin is disclosed in Document 2.
As described above, although it is effective to cover the surface of the reflecting layer with the corrosion resistant layer in order to suppress corrosion of the reflecting layer, there is a problem in that time required for production is prolonged since drying after application of the synthetic resin is required in the case where the corrosion resistant layer is formed by application of the synthetic resin. Also, since a solvent evaporates when the synthetic resin is dried, film thickness control of the reflecting layer becomes difficult, and bubbles are mixed in accompanying evaporation of the solvent, resulting in possible yield reduction.